The present invention relates to humidity or moisture sensing elements. The providing of suitable instruments for the measurement of relative humidity (RH) over wide ranges of RH (eg 10-90%) continues to be a challenging problem. While conventional humidity sensors have upper ohmic ranges of up to 10.sup.6 ohms a special electrode design and high impedance circuit in the present invention provide for routine measurement up to 10.sup.12 ohms. A conventional humidity measuring system is limited in the ability to measure values above 10.sup.6 ohms because of noise pickup. Our new design presents very little antenna effect because of the small surface of the sensor and all the leads are isolated. The nature of the present humidity sensor allows for the miniaturizing of the sensor without sacrificing lifetime and stability. Because the impedance is high, the electrical currents in the sensor are minute and the I.sup.2 R heating effect is very low. The sensor can thus be reduced in size. The sensor might be described as one having an extended impedance range in that while the impedance at high humidities (i.e. 90% RH) is in the order of 10.sup.3 ohms (this being similar to existing RH sensors), the impedance at low humidities (i.e. 10% RH) is in the order of 10.sup.12 ohms with a 10.sup.6 impedance at about 35% RH. Thus the improved sensor extends the range into the higher impedances moving up from prior devices at about 10.sup.6 to about 10.sup.12 ohms. Thus when referring to a high impedance sensing element herein we mean that at the low range of relative humidity (under very dry conditions), the resistance from conductor 11 to conductor 12 through the iron oxide is in the order of 10.sup.12 ohms.
In the prior art it has been known to use iron oxide in a humidity sensitive element as has been described in the article "Electrical properties of iron oxide polyethylene glycol humidity sensitive elements" by Nicholas, Pitkanen, Lavine, Zook and Hagen, May 1976, Journal of Applied Physics, Vol. 47, No. 5 Pages 2191-2199. That disclosure, however, was directed to a much lower impedance device, the polyethylene glycol being combined with the Fe.sub.2 O.sub.3 for the specific purpose of reducing the impedance.
When a high impedance device (10.sup.12 ohms) of the nature of the present invention is desired there are special problems to be overcome, one of which problems is that of leakage currents. The novel design of the present detector is specifically directed to overcome this problem. Thus the described structure is easy to make and yet easy to control stray leakage paths. The low impedance guarded structure reduces pickup by providing shielding. The compact structure provides low electrical pickup in that the small geometry results in small capacity to radiating electric fields.